And he did it with gusto! Here's part of one exchange:
WALLACE: But, Mr. Trump, Mr. Trump, your numbers don't add up. Please put up full-screen number four. The Education Department, you talk about cutting, the total budget for the education department is $78 billion.
And that includes Pell grants for low-income students and aid to states for special education. I assume you wouldn't cut those things. The entire budget for the EPA, the Environmental Protection Agency, $8 billion.
WALLACE: The deficit this year is $544 billion. That's more than a half trillion dollars. Your numbers don't add up, sir.
Wallace's instant, graphical fact-checks delighted other journalists, who quickly tweeted their appreciation.
Moments later, Wallace did the same thing when Trump touted the savings he could achieve by negotiating better prices on prescription drugs.
TRUMP: You are talking about hundreds of billions of dollars ...
WALLACE: No, you are not.
TRUMP: ... if we went out to the proper bid. Of course you are.
WALLACE: No, you're not, sir. Let's put up full-screen number two. You say that Medicare could save $300 billion a year negotiating lower drug prices. But Medicare total only spends $78 billion a year on drugs. Sir, that's the facts.
Will math dissuade Trump supporters? Nothing has, so the odds don't seem high. Still, Wallace's slide was an extremely vivid way of demonstrating that Trump's fantastical campaign promises aren't deliverable.
And by calling out the Manhattan billionaire with a bit of flair, Wallace produced a memorable TV moment that will likely have people talking about his fact-checks after the debate. To have any shot at being effective, the critique needs attention.
And if anyone has proven the value of attention in this campaign, it's Trump.